Such ballasts generally have a capacitive smoothing filter located between the rectified supply and the ballast itself. This smoothing capacitor causes harmonic distortion of the current waveform during periods in which the rectified output is higher than the voltage over the smoothing capacitor, and during which time the capacitor charges up. This charging time, or conduction angle, is very small if a large capacitor is used, and all the required charge has to be loaded into the capacitor in a short period of time. This results in a heavy current output from the rectified supply during the short conduction angle, and causes current spikes in the rectified supply.
These current spikes increase the harmonic content of the power supply, and when a number of ballasts are being used, this increased harmonic distortion causes a poor power factor in the supply. This position is frowned upon by electricity supply authorities, who generally specify the minimum power factor allowable, and/or the maximum harmonic distortion allowable.
One known solution to this problem, is to use an inductive and capacitive filter between the rectified supply and the ballast input, in order to quench the current spikes This requires the use of extremely large inductors, which are expensive and bulky.
It is also known to use a storage conversion principle, whereby an inductor is controlled at a high frequency in order to allow charging of the smoothing capacitor over a wide conduction angle. This system however requires a control circuit for the storage converter, in order to regulate the discharge of current from the storage inductor.
Such a use of the storage conversion principle is described in South African Patent No. 81/2504 in the name of Siemens Aktiengesellschaft.
The circuit is relatively complex and could be expensive to produce.
It is an object of this invention to provide a method and apparatus for the reduction of harmonics in gas discharge lamp ballasts.